Clear containers made from polymeric materials, such as deli containers, are highly desired by food producers and packagers, as well as consumers that use containers to store food and other materials/goods. These polymeric materials are typically standard thermoforming polypropylene materials that are low melt flow polymers blended with additives such as antioxidants to help with reworking unused plastic (“regrind”) back into the process. The antioxidants help maintain color and melt flow of the material, and by adjusting the level of antioxidants, the color, clarity and haze of the material is affected. It is desirable to have containers that are not discolored or hazy.
Historically, thermoforming polypropylene has been difficult, compared to thermoforming other polymers such as polyvinylchloride. The temperature range in which polypropylene can be thermoformed is smaller than that for other polymers. In addition, it is standard practice to use various additive packages which typically included phosphite antioxidants to reduce discoloration and improve haze.
The current methods of improving clarity and reducing discoloration, and in particular “yellow” or “brown” haze, in clear polymer containers are to reduce process temperatures and use different nucleating agents. Readjusting processing conditions and testing or adjusting nucleating agents and additive packages require time and added costs for both polymer manufacturers and end-users.
For example, European Patent Application 1211289A1 discloses additional clarifying or nucleating agents for the reduction of haze levels after injection molding and thermoforming. U.S. Pat. No. 4,692,489 provides for blending a random copolymer resin with an antioxidant stabilizer, a primary neutralizer, and various other additives, under a nitrogen atmosphere in order to improve haze and yellowness. However, the percent haze in the resins generated was lowered only to between 38 and 40 percent, still a high value.
It would therefore be desirable to provide an improved clarity, low haze, low melt flow polymer that requires fewer additives and no major required changes or steps at the processing end.